/u/J-Force has previously answered Were the Assassins really prolific enough as assassins to deserve being the eponym of the same, or were they victims of character assassination?

/u/Talleyrayrand has previously answered Did professional assassins exist in medieval Europe? If so, where would they be trained? How would one become an assassin?

/u/mmjm123 has previously written about assassins/Templars during the Crusades.

EDIT: Someone else might have an original answer for you. These are places to start.